Coal and other sedimentary deposits exist in nature in generally parallel layers or seams of considerable length and breadth. Often the seams are bounded above and below by layers of other materials such as rock, shale, sandstone, and other materials. In mining, it is obviously desirable to remove just the useful deposits, such as coal, without simultaneously removing rock or any other contaminant.
One technique for removing coal from seams is called the longwall method where an excavating machine traverses back and forth along the wall as it removes material from the wall. The coal is actually removed from the seam by rotary members employing cutters which scrape, break, and otherwise loosen the coal from the seam from which it is automatically conveyed to the surface of the mine. A coal seam is not necessarily flat nor of uniform vertical dimension. Seams are frequently found to undulate and vary in height.
It has long been recognized that there is a need for some sort of sensor to "read" the characteristics of the seam in order to control the vertical movement of the cutters as the machines traverse the seam. Many attempts have been made to develop automatic sensing devices whereby machines may be controlled by operators remote from the machine in response to signals or "readings" taken by the machine itself.
One example of a long wall mining machine is shown in U.S. Pat. No. 4,143,552 to Godfrey. The machine is moved lengthwise of the seam on tracks and has a ranging arm mounting a rotatable cutting member or cutter head for heightwise adjustment as well as rotation. The cutting member mounts cutters at its periphery which remove the coal from the seam.
The patent discloses a vibration transducer in the form of one or more accelerometers mounted on the cutter support or ranging arm to sense sonic vibrations and convert them into electric signals. While this type of device does sense vibration, being mounted on the support arm, it also picks up the vibration from all of the cutters causing impure signals to be produced and cannot locate the source of the vibration.
To eliminate this problem, various attempts were made to sensitize only a single cutter in a rotary member employing a plurality of cutters. Such cutters were initially employed to detect the barrier between coal and rock. However, such barriers are not always distinct and misleading signals were obtained. Furthermore, it is often necessary to leave a layer of coal adjacent the roof or top of the seam as well as on the bottom or floor. But cutters which can only detect a coal rock interface are inadequate for this purpose.
Another problem with sensitized cutters employed heretofor, was that they were equipped with strain gages or accelerometers, which, for the most part, were not durable enough to be employed in commercial applications.
Attempts were made to transmit cutter signals from the rotating member to the non-rotating machine frame or ranging arm by way of electrical slip rings and/or battery powered radio transmitters. Due to mine safety requirements, such electrical equipment, could not, for one reason or another, comply with safety regulations and slip rings could not withstand the rigors of mining operations or the continuous presence of dirt and dust. Accordingly the apparatus did not meet with commercial success.
Accordingly, the present invention has as one of its objectives to provide apparatus for producing signals from a mineral seam which is not dependent upon engagement with the boundary of a seam and other sedimentary layers, e.g., a coal-rock interface.
It is another objective of this invention that means be provided to transmit signals from a rotary cutter to a stationary portion of the machine frame while complying with mine safety requirements and not employing equipment that cannot withstand the rigors of a mining environment.
It is still another objective of this invention that any equipment employed with the replaceable cutters of a commercial rotary coal mining machine be simple and not interfere with the process of replacing cutters so that ordinary mine personnel may replace worn cutters with ordinary, standard new cutters without interfering with the signal detectors.